1. Field of the Invention
The present invention relates to a method and apparatus capable of detecting spherical aberration caused by a storage medium, and more particularly, to a method and apparatus capable of detecting and compensating spherical aberration caused by a storage medium using a diffractive optical element.
2. Description of the Prior Art
With rapid development of technology, optical systems used in large-size and high-resolution digital displays (such as projectors or projecting televisions), compact disc (CD) players, digital versatile disc (DVD) players, and the like also progress rapidly. Aberrations can downgrade the overall performance of optical systems. Therefore, it is very important to reduce aberrations in optical systems.
With increasing demands for larger storage capacity, traditional CDs and DVDs no longer satisfy user requirements. Blu-ray discs (BD) and high density digital versatile discs (HD-DVD) providing large data storage size have become future trends. The numerical apertures (NA) of object lenses used in a BD player and a HD-DVD player is 0.85 and 0.65, respectively, each of which is much larger than the 0.45 NA object lenses of a traditional CD player. Also, the BDs and the HD-DVDs have multi-layer structures and can thus provide larger storage capacity.
Certain major aberrations in an optical system are spherical aberration, coma aberration, astigmatism, curvature of field, etc. An ideal lens is theoretically capable of focusing light onto a single point. However, spherical aberration occurs when an actual lens fails to focus incident light onto a single point. Usually, the focus of a lens is designed based on the thickness of a storage medium, so that laser light can be focused onto the surface of the storage medium for data access. When the thickness of the storage medium deviates from its nominal value due to process variations or other reasons, or the storage medium has irregular thickness, spherical aberration will occur, resulting in blurred and unrecognizable images. The value SA of spherical aberration can be represented by the following formula:SA≅λ*(NA)4/δ
Using a CD player as an example, λ is the wavelength of an incident laser, NA is the numerical aperture of an optical pick-up of the CD player, and δ is the thickness variation of the disk substrate. Since BD players and HD-DVD players use object lenses having larger numerical apertures and multi-layer discs result in larger thickness variations, it is often required to design an extra adjusting device for compensating spherical aberration.
Conventionally, a relay lens set is disposed in the prior art optical systems. By adjusting two lenses of the relay lens set, spherical aberration caused by thickness variations can be compensated. In order to accurately control the relay lens set, spherical aberration error (SAE) signals corresponding to the amount of spherical aberration when an optical system is accessing a storage medium has to be measured. The prior art optical systems compensate spherical aberration on a real-time basis. By adjusting the distance between the two lenses of the relay lens set during signal measurement, a best distance between the two lenses can be obtained when the measured signal has a maximum value (with minimum spherical aberration). This prior art method is ineffective since it requires accessing data from the storage medium for the calculating the spherical aberration error signals SAE. In another prior art optical system, a polarization beam splitter (PBS) is used for splitting light reflected by the storage medium into two light beams. The spherical aberration error signals SAE can then be calculated based on focus error signals (FES) of these two light beams. Since a PBS and a sensor is required, the prior art optical system is more expensive and complicated.